The present invention relates to a process for the production of nitric acid. The described process for the production of nitric acid is especially based on the mono-pressure or the dual-pressure process in which the ammonia feedstock is combusted with the aid of compressed process air and the nitrous gas formed during combustion is at least partly absorbed by water, thus forming nitric acid, and the non-absorbed tail gas is expanded from the second pressure to ambient pressure in a tail gas expansion turbine for generating compression energy.
The first step in the production of nitric acid is the reaction of ammonia NH3 with air yielding nitrogen oxide NO:4NH3+5O2→4NO+6H2O+907.3 kJ.
The nitrogen oxide NO thus obtained is then oxidised to form nitrogen dioxide NO2:2NO+O2→2NO2+113.1 kJ.
Finally, the nitrogen dioxide NO2 thus obtained is absorbed in water yielding nitric acid:4NO2+O2+2H2O→4 HNO3+256.9./.390.3 kJ.
In order to ensure that the water absorbs the maximum possible portion of the nitrogen dioxide NO2, the absorption is carried out at elevated pressure. The absorption pressure preferably ranges between 4 and 14 bar.
The oxygen required for the reaction of the ammonia feedstock is supplied in the form of atmospheric oxygen. The process air is compressed to a pressure which suits both the oxidation reaction and the absorption reaction.
The energy required for compressing the air is generated, on the one hand, by expanding the tail gas leaving the absorption unit to ambient pressure and, on the other hand, by utilising the heat dissipated in the reactions.
The designs of the different types of nitric-acid production plants are based on the specific requirements of the individual location.
Single-line nitric-acid plants are usually designed and rated for capacities between 100 and 1000 tonnes of nitric acid per day. If the size of the reaction unit is doubled, a single line can yield a daily production of up to 2000 tonnes.
Should a low daily production be required or should the energy prices prevailing at the location be comparatively low, the nitric-acid production plant will be designed on the basis of the high-pressure mono-pressure process. In this process, the pressure applied in the ammonia combustion and nitrogen oxide absorption units will be about equal, i.e. approx. 10 bar.
Should large rated capacities and/or higher acid concentrations be required, it will be more economical to base the design of the nitric-acid production plant on the dual-pressure process.
In the dual-pressure process, the ammonia is combusted at a first pressure which is lower than the absorption pressure. The nitrous gases formed during the combustion are cooled and then compressed to the level of the second pressure, i.e. the absorption pressure.
The formerly constructed plants using normal-pressure combustion and medium-pressure absorption have nowadays been replaced by plants using the more cost-effective mono-pressure or dual-pressure process. The nitric acid obtained is also referred to as sub-azeotropic nitric acid because, if such acid is distilled in a downstream distillation unit, the maximum nitric acid concentration that can be achieved will be 68% due to the formation of an azeotrope. The relevant literature describes a great variety of processes conceived to overcome this limit.
The end users of nitric acid, however, frequently wish to have a nitric acid of a concentration which is only slightly above such 68%, for instance when using nitric acid in the production of adipic acid, caprolactam, toluene diisocyanate or other substances that are nitrated with nitric acid. It is hence a long-standing need of industry to have available a cost-effective process for the production of nitric acid of a concentration between 68 and 76%.
The aim of the present invention, therefore, is to improve the conventional mono-pressure and dual-pressure processes for the production of sub-azeotropic nitric acid by simple and cost-effective means in order to permit the production of that nitric acid at a concentration of up to 76%.